Java, Immutability and Builders

When you have to work with complex nested object structures and you want an easy way to create them and test them, I think it's definitely worthwhile to create a builder. A builder is a fluent DSL to create (domain) objects. Below you can see how that works out

	package com.pelssers.domain;
	public class Person {
	private String firstName;
	private String lastName;
	public String getFirstName() {
	return firstName;
	}
	public void setFirstName(String firstName) {
	this.firstName = firstName;
	}
	public String getLastName() {
	return lastName;
	}
	public void setLastName(String lastName) {
	this.lastName = lastName;
	}
	}

view raw gistfile1.java hosted with ❤ by GitHub

	package com.pelssers.domain;
	public class PersonBuilder {
	private String firstName;
	private String lastName;
	/**
	* @param person the source person
	* @return a new personbuilder object with all properties from person copied
	*/
	public static PersonBuilder from(Person person) {
	return
	new PersonBuilder()
	.withFirstName(person.getFirstName())
	.withLastName(person.getLastName());
	}
	public Person build() {
	Person person = new Person();
	person.setFirstName(firstName);
	person.setLastName(lastName);
	return person;
	}
	public PersonBuilder withFirstName(String firstName) {
	this.firstName = firstName;
	return this;
	}
	public PersonBuilder withLastName(String lastName) {
	this.lastName = lastName;
	return this;
	}
	}

view raw gistfile1.java hosted with ❤ by GitHub

	package com.pelssers.domain;
	import org.junit.Assert;
	import org.junit.Before;
	import org.junit.Test;
	public class PersonBuilderTest {
	private static final String PELSSERS = "Pelssers";
	private static final String ROBBY = "Robby";
	private static final String DAVY = "Davy";
	private static final String WILLIAMS = "Williams";
	private Person robby;
	private Person davy;
	@Before
	public void setUp() {
	robby =
	new PersonBuilder()
	.withFirstName(ROBBY)
	.withLastName(PELSSERS).build();
	davy =
	PersonBuilder
	.from(robby)
	.withFirstName(DAVY).build();
	}
	@Test
	public void testRobby() {
	Assert.assertEquals(PELSSERS, robby.getLastName());
	Assert.assertEquals(ROBBY, robby.getFirstName());
	}
	@Test
	public void testDavy() {
	Assert.assertEquals(PELSSERS, davy.getLastName());
	Assert.assertEquals(DAVY, davy.getFirstName());
	}
	@Test
	public void testFromCreatesNewObject() {
	//davy is build with a builder created from robby
	//now check if we change robby's lastname this does not affect davy
	robby.setLastName(WILLIAMS);
	Assert.assertEquals(WILLIAMS, robby.getLastName());
	Assert.assertEquals(PELSSERS, davy.getLastName());
	}
	}

view raw gistfile1.java hosted with ❤ by GitHub